From WO 98/53200 and WO 00/79128 it is known to provide a wind turbine blade shell of glass-fibre-reinforced polymer with a carbon fibre layer, whose electrically conducting properties may be utilised for heating of the blade for de-icing thereof. The carbon fibre layer may be embedded in the fibre glass laminate.
From WO 00/14405 it is known to reinforce a wind turbine blade of fibre glass polymer with longitudinal strips of carbon fibre-reinforced polymer. The same publication discloses so-called hybrid composite materials, in which a mixture of glass fibres and carbon fibres has been used as fibre reinforcement.
U.S. Pat. No. 6,287,122 discloses the manufacture of elongated composite products, wherein a variation in the stiffness of the product along its length is obtained by altering the fibre content or the angle orientation of braided fibres.
U.S. Pat. No. 5,520,532 discloses a mould part of fibre-reinforced polymer of a varying stiffness, said stiffness being obtained by varying the number of fibre mat layers.
U.S. Pat. No. 4,077,740 discloses a helicopter rotor blade of a fibre composite material, the stiffness of the blade varying when seen in longitudinal direction. This feature is obtained by varying the fibre orientation so as to obtain an enhanced vibration dampening.
The stiffness of a wind turbine blade of course depends on the shell thickness, the cross-sectional geometry and the material. The cross-sectional dimensions of the wind turbine blade and the shell thickness vary in the longitudinal direction of the blade. Naturally, the largest cross-sectional dimensions are found at the blade root, where the blade cross-section often is substantially circular. Further along the blade, it adopts a more flat shape, which substantially corresponds to an ellipse.
As mentioned above, it is known to combine fibre types in the laminate to obtain the desired properties or compromises between the properties of the different fibre types as regards weight, stiffness and elongation at breakage. The construction of a blade having varying material properties in the longitudinal direction of the blade may, however, also be desirable. Carbon fibres are advantageous due to their stiffness and low density, but are on the other hand expensive compared to glass fibres. Consequently, it may be desirable to use carbon fibre reinforcement, where the use thereof is more advantageous. It may thus be advantageous to reinforce the outermost portion of the blade by carbon fibres and the innermost portion of the blade by glass fibres so as to reduce the weight in the outermost portion and thereby minimising the dead load moment. Less material and/or a smaller cross section is thus required at the innermost portion of the blade and the load on the turbine hub is reduced. The outermost portion of the blade may further-more be provided with an increased stiffness, whereby the risk of the blade deflecting so heavily that the blade tip hits the turbine tower is reduced. Problems with high dead load and insufficient stiffness have increased in recent years, as the length of wind turbine blades has increased continuously. This tendency seems to continue in the future.
In order to reduce the size of mounting flanges and the like, a demand may arise for small cross-sectional dimensions at the blade root. The total weight of the blade may be considerably reduced by using carbon fibres as reinforcement material for the innermost portion of the blade, ie for the blade root.
Other types of fibres, eg cellulosed-based fibres such as hemp fibres or flax fibres are potential materials for the reinforcement of wind turbine blades.
Other reasons may also exist for providing different positions on wind turbine blades with different types of reinforcement fibres. If two zones of a wind turbine blade, which are juxtaposed in the longitudinal direction, are reinforced with fibre types differing from each other and having differing stiffness and elongation at breakage, the blade is provided with an abrupt change in the stiffness. At heavy dynamic or static loads, most of the stress is received in the outermost portions of the stiffest fibres resulting in a high risk of these fibres and thus the blade being destroyed. Put differently, a deflection of the blade causes a heavy stress concentration at the boundary surface between the two zones in the zone having the stiffest fibres. The problem is particularly severe at dynamic loads to which the wind turbine blades are subjected.